1. Field of the Invention
This invention relates to a silicone rubber composition having improved crepe hardening and shelf stability.
2. Prior Art
It is known in the art that silicone polymers can be improved in physical properties such as tensile strength, tear strength, elongation, and heat resistance by blending large amounts of silica filler. However, in order to blend large amounts of silica filler in silicone polymers, large amounts of a wetter or dispersant must be added for improving the dispersibility of the silica filler.
One commonly used wetter is .alpha.,.omega.-siloxane diol represented by the formula: HO[(CH.sub.3).sub.2 SiO].sub.x H wherein x is a number of 10 to 20 and having a degree of polymerization of 10 to 20 as disclosed in U.S. Pat. No. 3,799,962. Also known is .alpha.-alkoxy-.omega.-siloxanol of the formula: RO[(CH.sub.3).sub.2 SiO].sub.y H wherein R is a short chain alkyl group and y is a number of 3 to 5. Further an anti-structure agent in the form of a mixture of hexamethyltrisiloxane diol and methoxyhexamethyltrisiloxanol is proposed in U.S. Pat. No. 3,925,285.
However, these prior art wetters suffer from a tacky problem. If the wetter is added in relatively large amounts for reducing crepe hardening, the resulting silicone rubber composition becomes sticky and difficult to work with and cures into products having poor mechanical properties. If the amount of wetter is restricted for preventing the compositions from becoming sticky, the composition becomes tack-free, but undergoes substantial crepe hardening. There is a desire to have a wetter which can minimize crepe hardening, eliminate stickiness, and improve the mechanical properties of cured products.
More particularly, when a silicone rubber composition is compounded, silica and the polymer are in wet contact. If the composition is stored in an unvulcanized state, pseudo crosslinking takes place between silica and the polymer due to hydrogen bonds and physical bonds by van der Waals forces. This phenomenon is known as crepe hardening. Once crepe hardening occurs, the composition turns to be very difficult to work with. The original state can be recovered by fully milling the composition with sufficient shearing forces. Of course, such additional milling is commercially disadvantageous.
Prior art wetters have a problem with respect to the compression set and mechanical strength of silicone rubber which has been vulcanized with vulcanizing agents added.
A further problem is that prior art wetters fail to provide transparent silicone rubber.